Associated Smithsonian Expert: Peter Wagner, Ph.D.

Peter Wagner (in the front) collecting snail fossils from middle Ordovician rocks in Death Valley, California

Courtesy of Shanan Peters, University of Wisconsin

Dr. Peter Wagner is a paleontologist and curator of Paleozoic mollusks at the Smithsonian National Museum of Natural History. He got his start studying geology at Michigan State University. He went on to earn a master's degree in Geological Sciences at Michigan State University, where his thesis focused on relationships among some Ordovician gastropods. As a doctoral student at the University of Chicago, Peter studied rates and trends of morphologic evolution as well as phylogenetic diversity of early gastropods. He served as a curator at the Field Museum of Natural History for more than a decade before joining the Smithsonian. His current research focuses on models of morphologic evolution, and how they affect our ideas of relationships and diversification among Paleozoic mollusks, particularly gastropods.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Painting of straight-shelled ammonite called baculites common during the Cretaceous
Photo by Mary Parrish, Smithsonian, National Museum of Natural History

About Cephalopods (Class Cephalopoda): Paleobiology

Compared to modern cephalopods, the fossil record reveals 20 times the number of species. The first cephalopods appeared during the late Cambrian (about 500 million years ago). They looked like squid, but with long, cone-shaped shells from which they extended their tentacles. Typically, the shells were divided into chambers (septa), added on as the animal grew. Some early cephalopods were enormous, as long as a school bus. For animals without backbones (invertebrates), they were sophisticated, with nervous systems and the ability to swim by jet propulsion. By the end of the Paleozoic (250 million years ago), cephalopods had diversified to include coiled shells like a modern nautilus. While the extinction event at the end of the Paleozoic impacted many species, cephalopods flourished again during the Triassic and Jurassic. The extinction event at the end of the Cretaceous (65 million years ago) left only the nautiloids and the coleoids (octopus and squid).

Fossil ammonoid (Imitoceras oweni) from Texas, dating to the Mississippian
Photo by Smithsonian Institution, Department of Paleobiology

About Ammonoids (Class Ammonoidea): Paleobiology

The earliest ammonoids on Earth, small organisms called Bacrites, had straight shells. Later ammonites had coiled shells made up of a spiraling series of chambers. They got their name from early Romans who mistook their fossils for ram?s horns (Ammon= a god with ram's horns). Ammonites became common in the seas of the Jurassic (200 million years ago). They were top predators, feeding on fishes, mollusks, arthropods, or other ocean creatures. Ammonites used good eyesight and tentacles to capture prey and feed it into a strong, crushing beak. Their closest living relatives are probably the modern nautiloids, but ancient ammonoids could be wider than a minivan. During the Cretaceous (about 150 million years ago), as ammonoids reached their heyday, some evolved to have shells that were coiled less or not at all. All ammonoids went extinct in the huge extinction event at the end of the Cretaceous (65 million years ago).